1. Field of the Invention
The present invention generally relates to a method of processing mud pulse telemetry and, more specifically, to a method of analyzing mud pulse telemetry signals using adaptive noise cancellation techniques.
2. Description of the Related Art
Typical petroleum drilling operations employ a number of techniques to gather information such as the size and direction of a bore hole and the types of materials through which a drillpipe and drill bit are drilling. Originally, the drillpipe and drill bit needed to be pulled from the bore hole and then instruments inserted into the hole in order to collect information about down hole conditions. This technique, or "wireline logging," is expensive in terms of both money and time; so techniques called Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD) were developed. LWD collects the same type of information as wireline logging while MWD also enables a driller to determine the direction of a bore hole during the drilling operation so that the driller can more accurately control the drilling operations. The techniques of the disclosed embodiment apply to both MWD and LWD and, for the purpose of the disclosed embodiment, they will be referred to together as "MWD/LWD."
A problem common to MWD/LWD is how to transmit data from the bottom of a bore hole to a point on the surface where it can be collected and processed. A typical technique for this type of data transmission is mud pulse telemetry. During the drilling operation, drilling mud is pumped from a mud pump downward through the drillpipe and emerges near the drill bit at the bottom of the drill hole. This mud cools and lubricates the drill bit, carries rock cuttings to the surface where they can be analyzed and prevents the walls of the bore hole from collapsing. In mud pulse telemetry, a transmission device, or "pulser," such as an electo-mechanical pulser or a mud siren near the drill bit generates a signal that is transmitted upward to the surface through the downward traveling column of mud. A transducer, typically at the surface, receives the signal and transmits it to a signal processor. The signal processor then decodes and analyzes the signal to provide real-time information about the drilling operation to the driller.
One problem with decoding and analyzing the signal is that noise seen by the transducer, generated by the drilling operation, obscures the signal. There are a number of potential sources of noise generated during MWD/LWD. Noise may be introduced by the turning of the drill bit and drillpipe and/or from the mud pump used to force the mud into the drillpipe. Another source of noise is a reflected signal that is created when the original signal hits a pulsation dampener, or "desurger," near the top of the mud column and is reflected back down the hole. In addition to noise, the MWD/LWD signal may be degraded by the type of mud, the mud pressure, the length and joints of the drillpipe, and the desurger.
To obtain reliable MWD/LWD signal decoding, slow data transmission rates are typically used (about 1 bit per second) in order to sustain reasonable signal amplitude, and therefore, achieve an acceptable signal-to-noise (S/N) ratio. If data transmission rates are increased, clock tracking and timing recovery, signal amplitude, and the S/N ratio between the pulser and transducer become very sensitive and difficult to maintain due to the nature of the drilling operations, thus, decreasing the reliability of the MWD/LWD data.
Recently, a technique that employs two transducers has been developed to ameliorate the effects of the noise sources on the signal. In general, the signal at a second transducer is subtracted from the signal at a first transducer. Specifically, the first transducer is placed such that a leading edge of an upward traveling signal can be sampled before a downward traveling signal caused by the reflection of the upward traveling signal arrives at the first transducer. The second transducer is placed either close to or at the point where the upward traveling signal is reflected and thus is, its output in essence, the upward traveling signal uncontaminated by the downward traveling reflected signal. One or both of the signals received at the first transducer and the second transducer are time shifted, and the second signal is then subtracted from the first signal. This technique produces a processed signal with more sharply defined leading and trailing edges. Because the information carried by a signal is typically encoded either in the pulse position or the timing and the phase of the signal, more sharply defined leading and trailing edges enable the processed signal to be less obscured by the noise and more easily decoded than a signal in a single-transducer MWD/LWD system.